The wind tunnel balance or wind tunnel test stand described herein and the associated method allow a quick conversion of the wind tunnel balance, for example, from a five-belt configuration to a one-belt configuration and vice versa. The wind tunnel balance described herein is, in particular, configured such that the conversion may be performed with an exchange of only a few wind tunnel balance components and with high process reliability and accuracy. The measurement accuracy of the wind tunnel test stand is very high in all configurations, in particular in one-belt and three-belt configurations, and even the smallest forces of, e.g., a few Newton may be reliably detected.
During the development of motor vehicles such as passenger cars or racing cars, these cars are usually tested in wind tunnels with regards to their aerodynamic properties. In order to measure forces acting on the vehicle in these aerodynamic tests, different test stands and wind tunnel balances were used up to now depending on a test target and a type of the vehicle. In wind tunnel balances, a realistic air flow is blown onto the vehicle by a large fan. In order to form it in an aerodynamically realistic manner, the vehicle underbody has to move at wind speed.
Known test stand types have, for example, a different number of belt units, and accordingly one-belt, three-belt, and five-belt test stands are distinguished. The coordinate system of the test stands is usually selected such that the X-axis extends in the direction of travel of the conveyor belt of the test stand or wind tunnel balance. The Y-axis is transverse to the conveyor belt. The Z-axis extends from the surface of an upper run of the conveyor belt away from the conveyor belt. This coordinate system is also used in the figures described below.
In the so-called five-belt systems, the flow at the vehicle underbody is simulated by a so-called middle conveyor belt or middle belt. Herein, the middle conveyor belt here only so wide that the vehicle wheels and the supports (rocker supports or rocker panels) do not rest on the middle band. The wheels of the vehicle rest on up separate wheel belt units which are as compact as possible.
In other words, a middle belt unit and, laterally thereof, four smaller (wheel) belt units are provided in a five-belt test stand. Usually, a motor vehicle rests on the four side belt units and the belt units are weighed.
Five-belt test stands are preferably used in measuring and testing passenger cars, since frequently mounting motor vehicles on a five-belt test stand is less time consuming than on one-belt test stands. One-belt test stands are particularly preferred for motor vehicles with high output powers, such as racing cars, which usually generate much higher output powers than passenger cars or series-production vehicles.
In competition vehicles (sports or racing cars), the complete vehicle or a model of the vehicle is placed on a single large conveyor belt. This configuration is typically referred to as a one-belt test stand (configuration) or a single belt test stand. The conveyor belt of the one-belt test stand is wider than the (maximum) width of the vehicle and longitudinally extends beyond the vehicle, in particular rearward of the vehicle.
In other words, one-belt test stands usually have a belt unit with a wide continuous conveyor belt which rotates on two rollers or drums. For aerodynamic tests, the motor vehicle is positioned with all wheels on this single conveyor belt (therefore one-belt test stand) and fixed with respect thereto. This is accomplished, for example, with fixing devices that are situated to the side of the conveyor belt. The wheels may be supported on mounts which may be positioned underneath the upper run of the conveyor belt, between the rollers. The mounts may include weighing pads or the like, such that Z-forces may be detected through the belt. (In this context, this is referred to as a through-belt measurement)
In particular, one-belt test stands are the preferred choice in measuring and testing racing cars, since, inter alia, an accurate picture of output powers can be obtained that are of particularly great importance in racing cars.
It should be noted that the through-belt measurement described above may also be applied to three-belt, five-belt, and seven-belt test stands. In three-belt test stands, the tires of the vehicle rest on narrower side belt units. To simulate the driving surface, a so-called middle belt runs between the conveyor belts of the side belt units. The through-belt measurement takes place at the two side belt units. In five-belt test stands, the through-belt measurement is used, in particular, when the track width of a vehicle is less than or equal to the width of the middle belt, and the motor vehicle only rests on the middle band.
Various motor vehicle manufacturers are engaged in producing both series production vehicles and competition vehicles. As the construction of a wind tunnel with high-performance fans entails high cost, there is a desire for a convertible/retrofittable test stand which allows for a quick conversion from a so-called five-belt system to a “single-belt system” or three-belt system. Herein, it is also conceivable that the single-belt system consists of several sub-belts.
DE 10 2008 036 315 B4 describes a wind tunnel test stand which is convertible from a five-belt configuration to a three-belt configuration. On the weighing platform described therein, wheel belt units may be installed, which, along with a fixedly installed middle conveyor belt, constitute a five-belt configuration. The four wheel belt units may be removed and two single lateral conveyor belts may be arranged on the weighing platform.
Since the large and heavy side belts are mounted on the weighing platform, the weighing platform must bear large weights. However, this reduces the accuracy of the weighing platform and the large mass that weighs on the weighing platform reduces the natural resonance of the balance undesirably. Another very significant disadvantage is the fact that parasitic forces or fictitious forces and rolling resistances of the wheels cannot be separated from aerodynamic forces on the side belts.
In DE 10 2011 085 640, a one-belt wind tunnel test stand is described which makes it possible, in particular, that fictitious forces (e.g., generated by the fixation of the vehicle) and rolling resistances of the wheels may be separated from aerodynamic forces. The conveyor belt or the one-belt unit is supported floatingly/slidably. A movement in the X- and/or Y-direction is supported on the weighing platform via force measuring means. The one-belt unit is decoupled from the weighing platform in the Z-direction with respect to force transmission, while such a force coupling is provided for the X- and/or Y-direction. However, a disadvantage of this wind tunnel test stand is that the system is not convertible, for example, from a one-belt to a five-belt configuration, or only convertible at great expenditure of time.